1. Field of the Invention
The present invention generally relates to a technology for embedding invisible digital watermark in color image data.
2. Description of the Related Art
Recent proliferation of color image forming apparatuses, such as printers and copying machines, has ironically made it easy to produce unauthorized duplication of a printed matter of a color image.
To this end, information embedding (digital watermark) technology has recently been used as a method for preventing unauthorized duplications. However, addition of a visible digital watermark to an image undesirably causes a change in the hue of the image at position of the watermark.
To this end, a digital watermarking technology that makes it possible to embed an invisible digital watermark in image data without causing degradation in image quality has been developed. Japanese Patent No. 3682382 discloses a technique of embedding special information corresponding to digital watermark data in a portion, such as a least significant bit, of image data where influence exerted by the embedding is relatively small. In this technique, image data in RGB color space first is converted to image data in YPbPr color space. Next, bits in only a to-be-watermarked area on a highest-luminance plane among luminance planes of the converted image data are shifted. Thereafter, binary image data is embedded in a bit plane that includes the to-be-watermarked area.
Japanese Patent Application Laid-open No. H4-294682 discloses a technique of adding an image signal corresponding to an output color component of a least noticeable color to a human eye (e.g., yellow). This technique allows embedding of a dot pattern or the like that represents watermark information in the least noticeable manner.
Japanese Patent Application Laid-open No. 2002-281283 discloses a technique of adding new color information to image data having luminance information and color information as follows. In this technique, input image data that is in a first color space is first converted into converted image data in a second color space. New color information that can be represented in the second color space is subsequently added to the converted image data. The converted image data is finally inversely converted into image data in the first color space as output image data.
However, the technique disclosed in Japanese Patent No. 3682382 is disadvantageous in that when image data in which digital watermark data is embedded by using this technique is subjected to low-pass filtering, pixel data on the significant least bit is lost. Moreover, because image compression is generally performed by removing such a portion of image data that affects image quality to a relatively small extent to reduce data amount, when the image data is subjected to such image processing as image compression, the digital watermark data can be lost. Accordingly, the watermark data can be relatively easily lost from the image data, which makes it difficult to detect the digital watermark data having undergone image processing.
The technique disclosed in Japanese Patent Application Laid-open No. H4-294682 is disadvantageous in that addition of a dot pattern that represents watermark information can result in degradation in image quality. Particularly, when the image signal represents an output color component that is not present in an original image, or the dot pattern is added to a lightly-and-uniformly-colored portion of the image, the dot pattern can be inappropriately visible and obtrusive.
The technique disclosed in Japanese Patent Application Laid-open No. 2002-281283 is disadvantageous in that when a value of the new color information to be added is equal to or below an intermediate value between a maximum value and a minimum value, which depends on a number of color tones within the range of the second color space, the act of adding new color information to image translates to adding or removing specific information from the color information of the image data. Accordingly, hue of an image reproduced based on the output image data can differs from its original image.